How to Write a Tree Expansion Listener

Sometimes when using a
tree, you might need to react when a branch becomes expanded or collapsed. For example, you might need to load or save data.

Two kinds of listeners report expansion and collapse occurrences: tree expansion listeners and tree-will-expand listeners. This page discusses tree expansion listeners. See How to Write a Tree-Will-Expand Listener for a description of Tree-Will-Expand listeners.

A tree expansion listener detects when an expansion or collapse has already occured. In general, you should implement a tree expansion listener unless you need to prevent an expansion or collapse from ocurring .

This example demonstrates a simple tree expansion listener. The text area at the bottom of the window displays a message every time a tree expansion event occurs. It's a straightforward, simple demo. To see a more interesting version that can veto expansions, see How to Write a Tree-Will-Expand Listener.